Most automotive vehicles include an alternator and some method for alerting the vehicle operator to the fact that the alternator is or is not developing an output voltage as required. In many cases, a "check engine" light or the like is energized when a sensor indicates that the alternator is not rotating and, therefore, not developing the required output voltage.
A common method for indirectly sensing alternator rotation is to measure a so-called "phase tap" voltage on a connection to one of the alternator windings. By comparing the phase tap voltage to a reference, a determination can be made as to whether the measured voltage is indicative of a properly rotating alternator.
In some applications, the phase tap voltage is differentiated and otherwise processed so as to isolate the AC output of the alternator from DC voltages. An appropriate AC output is then interpreted as an indication of a properly rotating alternator. Although this technique is reliable and is very acceptable for some applications, AC signal processing is too expensive for certain other applications.
Another technique avoids the use of AC signal processing by directly sensing a positive output voltage from the alternator, comparing the sensed voltage to a reference, and actuating a fault indicator (such as the "check engine" light) when the magnitude of the sensed voltage is less than the reference. Thus, either a DC or an AC signal of sufficient magnitude at the alternator's phase tap will inhibit the fault indicator.
Although the DC type signal processing described above can provide acceptable results under nominal conditions, it is subject to error. Specifically, the rectifier diodes, which are normally part of every alternator, have leakage currents associated with them. If those leakage currents reach a sufficient magnitude, they can develop a voltage at the alternator's phase tap which is large enough to be interpreted as an indication of a properly rotating alternator, even when the alternator is stopped. This condition is discussed in more detail below, but it is sufficient here to say that this DC type signal processing can indicate that the alternator is working properly when, in fact, it is not even rotating.